This invention relates to zoom viewfinders.
U.S. Pat. Nos. 4,725,130 and 4,854,680 each describe a 3-unit Reverse Galilean-type zoom viewfinder. The finders disclosed in the above-mentioned patents all have a 3 unit, minus-plus-minus configuration. In these finders, the positive lens unit always moves for zooming, while the two negative lens units generally are stationary. U.S. Pat. No. 4,725,130 also discloses that the first negative lens unit may also move for zooming.
The inherent problem with this approach is that when the above minus-plus-minus component configuration is used and only the single positive unit is moved for zooming, the zoom finder suffers from defocusing. To accommodate this defocus, the lens of the observer's eye has to constantly adjust itself as the viewfinder zooms. Although, as disclosed in U.S. Pat. No. 4,725,130, the movement of the first lens unit in conjunction with the second lens unit can rectify this problem, a design modification such as this would loose an advantage of having fixed (i.e. stationary) outer lens elements.
U.S. Ser. No. 07/783,588 shows a finder which solves many of these problems by having four lens units that are:
(a) a stationary first lens unit having negative refractive power; PA1 (b) a movable second lens unit having negative refractive power; PA1 (c) a movable third lens unit having positive refractive power; and PA1 (d) a stationary fourth lens element having negative refractive power, where the second and third units are being movable for zooming.
U.S. Pat. Nos. 4,725,130 and 4,854,680 do not identify the location of the field stop. If the field stop is assumed to be the clear aperture of the front most lens element, then the value of margin for the disclosed viewfinders will not remain constant throughout the zoom range. U.S. Ser. No. 07/783,588 does not address the question of margin control either.
Reverse Galilean-type viewfinders which include a projected frame or reticle easily delineate the field of view. When we eliminate the reticle or the frame, controlling the field of view becomes more difficult. Merely changing magnification does not guarantee that as we zoom we will see the correct field of view (FOV), because the aperture (window) that limits the FOV may appear to change size resulting in a variation in the apparent FOV. Having a fixed location field stop (i.e. framing window) with a variable aperture may solve this problem, but at the expense of the system's compactness.